Electrostatic MEMS resonators have been a promising technological candidate to replace conventional quartz crystal resonators due to the potential for smaller size, lower power consumption and low-cost silicon manufacturing. Such devices typically suffer, however, from unacceptably large motional-impedance (Rx). MEMS devices operating in the out-of-plane direction, i.e., a direction perpendicular to the plane defined by the substrate on which the device is formed, have the advantage of large transduction areas on the top and bottom surfaces, resulting in a reduction in motional-impedances. Consequently, out-of plane devices have received an increasing amount of attention resulting in significant advances in areas such as digital micro-mirror devices and interference modulators.
The potential benefit of out-of-plane electrodes is apparent upon consideration of the factors which influence the Rx. The equation which describes Rx is as follows:
            R      x        =                  c        r                    η        2              ;            with      ⁢                          ⁢      η        =                  V        ⁢                              ∂            C                                ∂            g                              =                                    ɛ            0                    ⁢          AV                          g          2                    wherein “cr” is the effective damping constant of the resonator,
“η” is the transduction efficiency,
“g” is the gap between electrodes,
“A” is the transduction area, and
“V” is the bias voltage.
For in-plane devices, “A” is defined as H×L, with “H” being the height of the in-plane component and “L” being the length of the in-plane component. Thus, η is a function of H/g and H/g is constrained by the etching aspect ratio which is typically limited to about 20:1. For out-of-plane devices, however, “A” is defined as L×W, with “W” being the width of the device. Accordingly, η is not a function of the height of the out-of-plane device. Rather, η is a function of (L×W)/g. Accordingly, the desired footprint of the device is the major factor in transduction efficiency. Out-of-plane devices thus have the capability of achieving significantly greater transduction efficiency compared to in-plane devices.
The encapsulation of the inertial sensor is a standard process, e.g., performed by waferbonding. This is needed in order to protect the sensor structure from environmental influences and in order to provide an optimal operation pressure. Accelerometers typically have a higher pressure (>10 mbar) in order to provide sufficient damping. Gyroscopes have a lower pressure (<10 mbar) in order to operate efficiently. The encapsulation process is not further described herein or shown in the figures.
Additionally, MEMS sensors are generally fabricated using dedicated process flows for each sensor with each sensor on a unique chip. For example, a pressure sensor is fabricated with a completely different process flow than an inertial sensor and, as a result, it is difficult to fabricate both sensors on a single chip.
What is needed is a device that is fabricated using commonly understood fabrication steps that combines multiple sensing devices of different types on a single chip. It would be beneficial if the device could be realized using a single fabrication process on one single chip.